Newly generated Rails projects will now come pre-configured to use locale files. Just drop your translated locale files in config/locales and you're ready to go.
Locales allow you to present your website in multiple languages. Instead of hard-coding all the text of your website, you can query the locale files for the text. This allows you to easily support multiple languages, give locale files to a translator for translation, etc. Good support for internationalization is important for any web framework like Rails.
For more information about internationalization on Rails projects, see the Rails I18n homepage.